1. Field of the Invention
The present invention relates generally to antennas, and more particularly, but not by way of limitation, to an antenna for communicating two independent microwave signals being orthogonally polarized.
2. Description of the Related Art
Local multipoint distribution systems (LMDS) are used for communicating information from a central location to distributed locations. Recent developments of data communication have demanded that high speed data communication be available between the distribution locations from the central location. For example, a new telecommunications company may wish to serve many customers without constructing cable to the premises of customers or renting existing cable from the current local telecommunications company. From a central antenna location, communication with multiple customers is possible. Use of a local multipoint distribution system generally has up to a three to five mile transmission range and may employ wavelengths of about one-centimeter or less.
In addition to LMDS systems, multichannel multipoint distribution systems (MMDS) are utilized to communicate, for example, television channels or data information from a central location to multiple distributed locations. MMDS systems have a longer range of communication, generally 35 miles, than LMDS systems, and employ wavelengths of about 15 cm.
While it is possible to create distribution channels for the LMDS and MMDS systems using fiber optic cables, installation of optical fiber cables is difficult and expensive due to construction and legal fees. To avoid the costs of using optical fiber or other cables, recent developments of wireless communications providing high speed service have caused LMDS systems to be preferred. Such wireless communications include using microwaves such as 30 GHz (i.e., wavelengths of about one-centimeter or less) and higher. This recent move toward using LMDS systems, however, have required the development of infrastructure, including special antennas, to support point-to-multipoint (and reverse) communication.
It is desirable to have constant power density received at the ground level without regard to the relative distance from the antenna. Because power density radiated from an antenna drops as 1/R2, where R is a range variable, it is therefore desirable to produce a cosecant-squared antenna radiation pattern in the elevation plane. One type of antenna that is capable of producing a cosecant-squared antenna radiation pattern in the elevation plane, and currently used in LMDS systems is a reflector antenna known as a hog-horn antenna having a specially-shaped reflector (situated between two parallel plates and illuminated with an offset feed horn). The reflector is generally not parabolic. For the LMDS systems, the antenna is generally mounted on a building or a tower to provide coverage over a ground sector or region.
As the antenna is mounted (see FIG. 7) at a height H, the following equation may be applied: sin(xcex8)=H/R, where xcex8 is the angle measured from the antenna to the ground from the horizon. As xcex8 varies from the horizon to approximately 45 degrees or less, R becomes smaller as 45 degrees is approached. Therefore, to produce an antenna radiation pattern that has constant power density at ground level, an antenna radiation pattern having a distribution of R2 will substantially negate the 1/R2 decrease in power density. A simple geometrical equation, R2=1/sin2 xcex8=csc2xcex8, thus shows that to produce an antenna having an elevation plane pattern that has an R2 distribution, a cosecant-squared elevation radiation pattern is desired.
As understood in the art, a hog-horn antenna can be made using a feed horn and a specially shaped (non-parabolic) reflector that produces a cosecant-squared antenna radiation pattern. Note: hog-horn antennas with a parabolic reflector are also used, but produce a pencil beam elevation plane pattern, not a cosecant-squared type. A pencil-beam pattern is not useable for cosecant squared applications because of the resulting narrow beam width in the elevation pattern and lack of elevation null filling. There are specific uses for such an antenna, such as where coverage of a very narrow strip is desired.
In the azimuth patterns, it is desirable to restrict the signal to a specific angular pattern. This sector antenna allows for reuse of the same frequencies from the same location. For example, two 90 degree sector antennas may be mounted in opposing directions with negligible, if any, interference.
While the ability for a hog-horn antenna with a specially-shaped (e.g., non-parabolic) reflector to produce a cosecant-squared antenna radiation pattern has been known for years, these antennas have been limited by their ability to communicate only in a single polarization (i.e., either horizontal or vertical polarization). By having communication capabilities over only a single polarization, bandwidth is limited to half of the bandwidth that is possible by using both polarizations. To use both polarizations in a present day communication system desiring the cosecant-squared antenna radiation pattern of the hog-horn antenna, two antennas are typically utilizedxe2x80x94each one configured in a different polarization. The principles of the present invention allow for use of both polarizations, either separately or simultaneously, by a single, hog-horn antenna.